(1) Field of the Invention
The present invention generally relates to a suspension control system for controlling a suspension of an automotive vehicle, and more particularly to a suspension control system in which a change of vibration of the vehicle due to the road surface condition is detected and the damping force characteristic of a shock absorber provided in the suspension is altered so that the vehicle vibration is suppressed.
(2) Description of the Related Art
There is known a suspension control apparatus directed to suppressing a vibration of the automotive vehicle which has a long term approximately equal to one second and causes a car sickness (see Japanese Laid-Open Patent Application No. 62-80111). In the proposed apparatus, a long-term vehicle vibration is detected on the basis of a change of height of the vehicle. When a long-term vehicle vibration is detected, the damping force of a shock absorber is increased so that a suspension is set to a hard state. In order to detect a long-term vehicle vibration, changes of the vehicle heights with respect to front and rear wheels are detected. Then, a situation is detected where a signal showing a change of the vehicle height with respect to the front wheel has a phase opposite to that for the rear wheel and the signals with respect to the front and rear wheels have frequencies of vibrations in the signals approximately equal to a sprung resonance frequency (about 1.0 [Hz]) and where the amplitudes of the vibrations are greater than a predetermined vibration level. When the above-mentioned situation is detected, it is concluded that the vehicle has a long-term vehicle vibration. The frequencies of the vibrations are calculated by measuring a half of one period of each vibration.
However, there is room for improvement in the proposed suspension control system. That is, the procedure for suppressing a long-term vehicle vibration is started after such a vibration is actually detected. Thus, a driver and/or a passenger will feel a delay of the suppression of long-term vehicle vibrations, particularly in a specific vehicle, such as a sport-type vehicle or having vehicle vibration characteristics. Further, the conventional suspension control system cannot sufficiently suppress the long-term vehicle vibrations in some vehicle running conditions. For example, long-term vehicle vibrations alter depending on the vehicle speed or the road surface condition.